The present invention relates to film spools.
More particularly, the invention relates to film developing spools, i.e. spools for use in the developing of photographic films.
Still more specifically, the invention relates to a novel film developing spool which eliminates developing errors that are inherent in the prior-art spools due to the construction thereof.
Film developing spools of the general type in question are, of course, already known. An example is the one disclosed in German Pat. No. 884,277 which has a core provided with two disk-shaped end flanges. The inwardly facing sides of these end flanges (i.e. the sides which face axially of the core towards one another) are each provided with a continuous spiral groove. The edges of the film strip to be developed are to enter these grooves so that the film is wound up about the core in spiral convolutions which are maintained spaced from one another by a specific distance, which is determined by the radial spacing of the convolutions of the spiral grooves. The two end flanges can be rotated relative to one another about the longitudinal axis of the core (i.e. about the axis of the spiral grooves); this permits insertion of the film strip into the spool by first placing the free end of the strip into the spiral convolutions and then turning the end flanges so that the strip becomes wound up about the core of the spool. A rachets or similar device is provided to aid in this insertion. A cut-out may also be provided (under simultaneous reduction of the guide-rib thickness at the outer sides of the end flanges) through which a user's fingers can gain access to the edges of the film strip being inserted; at the cut-out location the inner film guide ribs forming the spiral groove are then inwardly extended by a certain distance in order to assure that the film strip will still be properly guided in the vicinity of the cut-out location.
Actually, there are two possible ways of installing a film strip to be developed on this prior-art spool. One of these is the insertion "from outside in" as described above. The other is an insertion from "inside out", meaning that the leading end of the film strip is secured to the core (a clamping tongue or similar device may be provided for this purpose) and a device is provided for briefly squeezing the film strip together in transverse direction in order to make it narrow enough to enter the convolutions of the guide spirals.
Once spools of the kind here under discussion are loaded with film strips (and there may, of course, be more than one such strip to be loaded on each spool), one or more of these loaded spools are stacked into a drum which is then rotated and is maintained (usually by a waterbath) at the requisite developing temperature. Due to the rotation of the drum the spirally convoluted film strips on the spools keep dipping into and being pulled through the chemical developer solution; since this results in a rather sparing use of the solution, and hence a cost reduction, spools of the type under discussion are popular. Moreover, such spools admit to another manner of effecting the development of the film, namely stacking the spools in a vertical tank which is then filled with the developer up to the upper spool and thereafter successively reversed through 180.degree. until development is completed.
A serious problem with the prior-art spools of the kind under discussion is that marginal zones of the film strips are usually not properly developed, especially when the rotary-drum type of development is employed. These "developing errors" are quite generally unsatisfactory; they are, however, especially unacceptable when the film negative extends over the entire width of the film strip (as in the case of roll films) so that there is in effect no "margin" and any improper development along the film strip edge is observed as a defect in the actual picture itself. The reason for these developing errors is that the spiral groove guiding the edges of the film strip on the spool tends to "shade" the edge zones; which is to say, it prevents proper access of the developer to the edge zones and thus creates the developing error. It is a reasonable assumption that this problem could be counteracted by reducing the depth of the ribs forming the spiral grooves. Unfortunately, this assumption does not hold true because such a reduction would jeopardize proper insertion and retention of the film strip in the spiral grooves, eliminating the certainty that all film convolutions are held spaced from one another by a specific predetermined distance, irrespective of whether the film is inserted into the spool by the "outside in" or the "inside out" method described before.
Other types of prior-art film developing spools are known from e.g. German Allowed Application AS Nos. 1,090,518 and AS 1,191,228. In these spools the end flanges have the shape of a spoked wheel and the axially inner sides of the spokes are provided with teeth which are intended to capture and hold the margins of the inserted film strips. What is lacking in these constructions is the continuous spiral groove so that there is no positive guidance and support for the film strip during loading of the same. The risk resulting from this is that successive convolutions of the film strip will not enter between the therefor intended teeth on the spokes, so that some teeth may retain no portion of the film strip at all, whereas two or more convolutions of the film strip may be captured in close proximity (or even contact) with one another between a single set of teeth--with the resulting disadvantages during subsequent developing. Furthermore, the missing spiral groove makes it impossible to load a film strip by sliding its leading end between the end flanges and pushing inwardly and, finally, the insertion of the film strip by means of an auxiliary inserting device is unreliable with this type of spool.